The present invention relates to optical switches, and more particularly, to an improved cross-point switching element.
Optical fibers provide significantly higher data rates than electronic paths. However, effective utilization of the greater bandwidth inherent in optical signal paths requires optical cross-connect switches. In a typical telecommunications environment, the switching of signals between optical fibers utilizes an electrical cross-connect switch. The optical signals are first converted to electrical signals. After the electrical signals have been switched, the signals are again converted back to optical signals that are transmitted via the optical fibers. To achieve high throughput, the electrical cross-connect switches utilize highly parallel, and highly costly, switching arrangements. However, even with such parallel architectures, the cross-connect switches remain a bottleneck.
A number of optical cross-connect switches have been proposed; however, none of these has successfully filled the need for an inexpensive, reliable, optical cross-connect switch. One class of optical cross-connects depends on wavelength division multiplexing (WDM) to effect the switching. However, this type of system requires that the optical signals being switched have different wavelengths. In systems where the light signals are all at the same wavelength, this type of system requires the signals to be converted to the desired wavelength, switched, and then be re-converted to the original wavelength. This conversion process complicates the system and increases the cost.
A second type of optical cross-connect utilizes total internal reflection (TIR) switching elements. A TIR element consists of a waveguide with a switchable boundary. Light strikes the boundary at an angle. In the first state, the boundary separates two regions having substantially different indices of refraction. In this state, the incident angle is greater than the critical angle of TIR, the light is reflected off of the boundary and thus changes direction. In the second state, the two regions separated by the boundary have the same index of refraction and the light continues in a straight line through the boundary. The critical angle of TIR depends on the difference in the index of refraction of the two regions. To obtain a large change in direction, the region behind the boundary must be switchable between an index of refraction equal to that of the waveguide and an index of refraction that is markedly smaller than that of the waveguide.
One class of prior art TIR elements that provide a large change in index of refraction operates by mechanically changing the material behind the boundary. For example, U.S. Pat. No. 5,204,921, Kanai, et al. describes an optical cross-connect based on an array of cross-points in a waveguide. A groove at each cross-point may be switched xe2x80x9conxe2x80x9d or xe2x80x9coff,xe2x80x9d depending upon whether the groove is filled with an index-matching oil. The index-matching oil has a refractive index close to that of the waveguides. An optical signal transmitted through a waveguide is transmitted through the cross-point when the groove is filled with the matching oil, but the signal changes its direction at the cross-point through total internal reflection when the groove is empty. To change the cross-point switching arrangement, grooves must be filled or emptied. In the system taught in this patent, a xe2x80x9crobotxe2x80x9d fills and empties the grooves. This type of switch is too slow for many applications of interest.
A faster version of this type of TIR element is taught in U.S. Pat. No. 5,699,462, which is hereby incorporated by reference. The TIR taught in this patent utilizes thermal activation to displace liquid from a gap at the intersection of a first optical waveguide and a second optical waveguide. In this type of TIR element, a trench is cut through a waveguide. The trench is filled with an index-matching liquid. A bubble is generated at the cross-point by heating the index matching liquid with a localized heater. The bubble must be removed from the cross-point to switch the cross-point from the reflecting to the transmitting state and thus change the direction of the output optical signal.
Switches based on a gas-vapor transition have a number of problems. First, substantial amounts of power are required to generate the bubble by vaporizing the index matching liquid. Second, to maintain a vapor bubble at a cross-point, a large temperature gradient must be maintained. This temperature gradient induces thermal stress and reduces the working temperature range of the device. Third, these switches depend on power being present at all times. If power is removed from the device, all cross-points will eventually enter the transmitting state.
Broadly, it is the object of the present invention to provide an improved cross-point for use in cross-connect switches and the like.
It is a further object of the present invention to provide a cross-point that does not require the high temperature gradients and high power input of the prior art devices discussed above.
It is a still further object of the present invention to provide a cross-point that maintains its state if power is removed from the cross-point.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an optical switch constructed from first and second waveguides. The first and second waveguides have ends disposed across a gap such that light traversing the first waveguide enters the second waveguide when the gap is filled with a liquid having a first index of refraction, whereas light traversing the first waveguide is reflected by the gap when the gap is filled with a material having a second index of refraction that is substantially different from the first index of refraction. The gap is part of a trench that contains a liquid droplet made from a droplet material having the first index of refraction. The droplet is located in the trench and is movable between the first and second positions in the trench, the droplet filling the gap in the first position. The gap is filled with a material having the second index of refraction when the droplet is in the second position. The droplet may be moved using an electric field generated by a plurality of electrodes arranged such that an electrical potential applied between a first pair of the electrodes creates an electric field in a region of the trench containing the first position. The droplet can also be moved by differentially heating two edges of the droplet so as to create a net force on the droplet in a direction parallel to the direction of the trench. The heating can be accomplished by illuminating one edge of the droplet with light of a wavelength that is absorbed by the droplet material.